1. Field of the Invention
The present invention relates to a zoom lens mechanism for use in a camera, and more particularly, to a zoom lens mechanism incorporated in a so-called lens shutter zoom camera.
2. Description of the Prior Art
Lens shutter cameras are widespread among the general public because of their compactness and handiness. In recent years, a demand for lens shutter cameras having higher functions has been increasing, which has led to the development of a lens shutter camera of a type provided with a zoom lens mechanism.
Generally, a zoom lens mechanism includes a stationary barrel fixed to the camera body, and a single or two movable barrels fitted inside the stationary barrel. In the stationary or the movable lens barrel, a first lens unit and a second lens unit constituting a taking optical system are movably incorporated through a lens driving mechanism using a cam. The focal length is varied between the shortest and the longest focal length conditions by increasing or decreasing the movement amount of the movable barrel, and at least one of the first and the second lens units is moved to change the relative distance between the lens units, thereby performing focusing.
As lens shutter cameras in which such a zoom lens mechanism is incorporated, the following have already been put into practice use: a zoom camera of a two-barrel type having two barrels including one movable barrel where a single movable barrel is fitted inside a stationary barrel fixed to the camera body; and a zoom camera of a three-barrel type having three barrels including two movable barrels where two movable barrels are fitted inside a stationary barrel so as to be successively movable out.
The lens driving mechanism of the former camera is structured such that the first lens unit is driven by a lead to move straight and the second lens unit is moved out in accordance with the movement of the first lens unit by a differential motion of the first lens unit and the stationary barrel. The lens driving mechanism of the latter camera has two moving-out mechanisms for lens units. The first moving-out mechanism employs a mechanism to move out all the lens units by using cams, and the second moving-out mechanism employs a mechanism to move out the lens units by using a bottomed cam. The bottomed cam is provided with cam grooves for each of the first, the second and the third lens units.
In either of these conventional structures, however, it is extremely difficult to realize both a compactness and a high zoom ratio. Specifically, since the moving-out amount of the movable barrel should be large to increase the zoom ratio, the dimension of one movable barrel along the optical axis should be large in the three barrel structure like the latter one, not to mention in the two barrel structure like the former one.
For this reason, when a zoom lens mechanism having a high zoom ratio is mounted on a camera, it is difficult to contain within the thickness of the camera the stationary barrel into which the movable barrels are retreated. Even if a perfect collapsible camera could be realized, since the camera should be of great bulk with a great thickness, it would be impossible to realize compactness. Therefore, the size reduction of the cameras embodying the above-described conventional structures is made by restraining the zoom ratio to be low. Thus, according to the conventional structures, zoom mechanisms are realized which are functionally unsatisfactory compared to zoom lens mechanisms mounted on single-lens reflex cameras.